I'm about to build a 50 watt plexi amp head. I already have a combo amp that has a 1x12 16 ohm celestion greenback that sounds great, and it's plenty loud. I'd rather just reuse that, instead of having to build a new cab, and get new speakers, etc. I have no need for an expensive new cab, and certainly no need for a ridiculously loud 2x12 cab to get the wattage rating correct. That speaker is only rated at 20w, however. I wanted to get an attenuator for my 50w amp anyway. Could I just use an attenuator like the Weber Mass 100 to plug the 50w head into the 20 watt speaker? If so, how do I know I have it set at a safe value that will not exceed the speaker's rated power capacity?